1. Field of the Invention
The invention relates to pressure measurement in roll covers for industrial rolls and in particular to the use of fiber Bragg grating sensors for determining a pressure imposed on a roll cover.
2. Description of the Related Art
Rolls are used in industrial papermaking for guiding, drying, and pressing the fibrous web sheet, which is the elementary body for the paper production. Rolls are further used in papermaking machines as guiding rollers for wet, press and dryer felts. In some sections of a papermaking machine the fiber web is conveyed between two cooperating rolls, where it is pressed in the nip formed between these rolls. The properties of a paper processed from the fiber web do strongly depend on the pressure profiles present in the nip sections between the various cooperating rolls. Manufacturers of paper are therefore anxious to monitor and control the pressure profiles in these nip sections.
The nip pressure is typically monitored with sensors placed between the roll core and the roll cover or inside the roll cover. Radial forces, i.e. forces acting in the radial direction of a roll, are usually measured using piezoelectric or electro-mechanic sensors, which both produce a voltage indicative of their deformation upon being pressurized. Since paper machine rolls rotate at a high speed, the sensor signals are usually transmitted to a signal processing unit external to the roll by means of a radio transmitter.
Apart from electrical sensors also fiber optical sensors are used for monitoring the pressure conditions within a nip. Fiber optical sensors generally use a fiber optical waveguide as sensing element, whereby the strain exerted on the fiber is determined by the impact of the strain on the fiber's optical properties.
In conventional optical fibers the strain or bending induced variation in the intensity of light passing the fiber is used as a measurement signal. But since measurement signals obtained by these effects carry no information regarding the location of the signal's origin, it is not possible to determine the position where the optical properties of the fiber have been changed.
If also the point of origin of a measuring signal is of importance, optical fibers comprising several discernable measuring sections are preferred. In a fiber Bragg grating sensor a respective measuring section is formed by a Bragg grating located in the fiber core. A Bragg grating consists of a sequence of variations in the refractive index of the fiber core along the longitudinal direction of the optical fiber. Depending on the respective measurement problem, the distances between consecutive changes in the (typically two) refractive indices (so-called grating spacings) are constant or vary within one Bragg grating. Light passing the core of the optical fiber is partially reflected at each refractive index changeover, with the coefficient of reflection depending on the refractive indices involved and the wavelength of the light. Multiple reflections at a sequence of changeovers in the refractive index lead to either a constructive or destructive interference. Therefore, only one wavelength will be (at least partly) reflected, when the grating spacing of a Bragg grating measuring section is constant, and multiple wavelengths will be reflected, when the grating spacing within one measuring section varies. The wavelengths of the reflected light and the coefficient of reflectance achieved depend on the grating spacings used, the refractive indices involved and the grating length given due to the number of refractive index changeovers present in a measuring section.
When the measuring section, i.e. the section of the fiber containing the Bragg grating, is exposed to strain, the grating spacings change thereby causing a proportional shift in the wavelength of the light reflected at the grating. A measurable wavelength shift is only obtained when the Bragg grating section of an optical fiber is stretched or compressed along its longitudinal direction. Forces acting transverse to the fiber axis do not provoke a measurable change in the grating spacings but only minor Bragg wavelength shifts by photo-elastic effects. Fiber Bragg sensors are therefore primarily used as strain sensors and not as pressure or force sensors.
The pressure profile in the nip section of two cooperating rolls is practically described by the forces acting radially on the rolls. For measuring these forces directly, the Bragg grating of a fiber Bragg sensor would have to be oriented in a radial direction of the roll. A respective arrangement is not practical, since the grating length of a fiber Bragg grating is in the order of millimeters and thus too long to be used within a roll cover. Furthermore, the minimum-bending radius of an optical fiber is in the order of approximately one centimeter, thus rendering the total minimum height of the fiber with respect to the radial direction too long for practical applications. For the same reason of limited bending radius, a radial orientation of a fiber Bragg grating in the roll cover allows only one measuring section per fiber, so that a separate fiber is required for each measuring location.
Optical fiber sensors are therefore usually arranged to measure the hoop strain induced in a roll cover by the forces acting in the nip section. For detecting the hoop strain of a roll cover, the optical fiber is embedded within the roll cover or at the boundary between the roll cover and the roll core. An arrangement appropriate for determining the tangential strain in a roll cover is disclosed in European patent EP 1 392 917 B1, where preferably micro-bend fiber optic sensors are disposed along a helical, axial, circumferential, and a “somewhat random” configuration. The optical fiber configurations presented in European patent specification EP 0 809 507 B1 include spirals, waves, scattered and straight lines along the length of the roll parallel to the roll axis. When using a waveform like a wiggly line, the measuring sections of the fiber, e.g. the Bragg gratings, are oriented in the circumferential direction of the roll or have at least a component in that direction.
When using more than one measuring section within one Bragg sensor fiber, the measurement signals have to be assigned to their respective measuring section of origin. If the fiber of a fiber Bragg sensor is arranged in a helical configuration, each measuring section crosses the nip at a different angular position of the roll. The measuring section assignment may therefore be implemented using the rotation angle of the roll.
A further method of identifying the measuring section from which a certain light reflection originates is based on a determination of the time interval between the launching of a light pulse into the Bragg fiber and the detection of a light echo reflected from one of the Bragg gratings in the fiber. A respective time multiplexed fiber Bragg grating sensor arrangement is for instance disclosed in the patent specification U.S. Pat. No. 4,996,419.
Instead of time multiplexing, wavelength multiplexing can be used for identifying a measuring section giving rise to a certain measuring signal. An example for such a distributed, spatially resolving optical fiber strain gauge is disclosed in document U.S. Pat. No. 4,806,012. In the described Bragg fiber, the grating spacing of one Bragg grating differs to any grating spacing of another Bragg grating formed in the same fiber. Accordingly the basic wavelength of a light echo produced on one grating differs from that produced on each of the other gratings. In this context it is noted that the term “light echo” as used in this specification refers to the light reflected on a Bragg grating in a Bragg fiber. A Bragg fiber hereby refers to an optical fiber having one or more Bragg gratings formed within its fiber core. The term “basic wavelength” as used in this specification refers to the wavelength of a light echo produced with a Bragg grating not exposed to strain. The spacing between the basic wavelengths of the different Bragg gratings of a Bragg fiber is usually chosen longer than the wavelength shifts expected for the Bragg fiber when used as designed for.
Irrespective of the type of Bragg fiber used, a fiber Bragg sensor embedded in a roll cover will only allow to determine the deformation of the roll cover caused by the forces acting in the nip and not the radial forces affecting the roll within the nip area. Variations in the deformation of the cover along the length of a roll are small compared to the variation of the deformation in the circumferential direction of the roll, since the pressure difference along the length of the roll is typically much smaller than between the inside and the outside of the nip. A Bragg fiber arranged along the length of a roll and parallel to the roll axis will therefore produce only small shifts in the wavelength of light reflected at a Bragg grating if any, with the shift values being furthermore not indicative of the absolute value of the compressing forces present in the nip. To get an indication of the absolute values of the compressing forces in the nip, the Bragg gratings are oriented with a component showing towards the circumferential direction of the roll. But even this does not allow a reliable suggestion of the forces present between two corresponding rolls, since the relation between the cover deformation and the compressing forces is very complex due to the elasticity of the roll cover.
A further drawback of fiber Bragg sensors is the limited number of discernible measuring sections, which can be arranged within one fiber. A Bragg fiber sensor consists of usually not more than between ten and twenty-five gratings, which will limit the density of measuring points available for determining the pressure profile in the nip.
What is needed in the art is to provide an improved optical fiber Bragg sensing system for the characterization of pressure profiles in a nip section of two cooperating rolls.